Such electrochemical three-electrode carbon monoxide sensors, which are already available commercially, are sensitive not only to the gas to be measured, i.e., carbon monoxide (CO), but also to molecular hydrogen (H.sub.2), which is contained particularly in flue gases, for example. This is referred to as "hydrogen cross-sensitivity" of the carbon monoxide sensor.
This hydrogen cross-sensitivity is due to the fact that at the electrodes, which are made of platinum black, not only the oxidation of carbon monoxide, but also that of hydrogen occurs very rapidly. In the case of commercially available carbon monoxide sensors, the cross-sensitivity is 30% to 100%, referred to the carbon monoxide sensitivity.
Electrochemically, the hydrogen cross-sensitivity is due to the fact that the oxidation of carbon monoxide on platinum occurs sufficiently rapidly only at a working electrode potential of approximately 800 mV to 900 mV, referred to a standard hydrogen electrode, since a certain oxygenation of the platinum surface is necessary for the carbon monoxide oxidation. By contrast, hydrogen oxidation occurs ideally already at 0 mV and actually in response to small overvoltages in the range of 0 mV to 200 mV, referred to the aforementioned standard hydrogen electrode.
Thus, by suitable choice of the electrochemical potential, hydrogen can, in the ideal case, be measured at pure platinum electrodes cross-sensitivity free in addition to carbon monoxide, while the hydrogen signal is always superimposed on the carbon monoxide signal. With commercially available electrochemical three-electrode carbon monoxide sensors, therefore, carbon monoxide is not measurable hydrogen cross-sensitivity free at a constant working potential (approximately 1 V with respect to the standard hydrogen electrode).
Besides being used for flue gas analysis, carbon monoxide sensors are employed to monitor air quality in basement garages and tunnels and to detect smoldering fires.
In Germany, performance tests of flue gas analyzers must be carried out in accordance with a guideline of the Central Association of the (German) Chimney Sweepers' Guild. According to that guideline, the measurement uncertainty in the measuring range greater than 400 ppm carbon monoxide must not exceed .+-.5% of the measured value. This means that the hydrogen cross-sensitivity of such carbon monoxide sensors used for flue gas measurements must be less than 5%, referred to the carbon monoxide main sensitivity of such sensors. However, this value is not attainable with conventional three-electrode carbon monoxide sensors.
Therefore, a compound gas sensor with a carbon monoxide measuring portion and a hydrogen measuring portion is on the market. The output of the hydrogen measuring portion serves to electronically compensate for the hydrogen cross-sensitivity of the carbon monoxide measuring portion. This compound gas sensor has, in addition to the three electrodes, a second working electrode for measuring hydrogen, which, as viewed from the gas inlet, is disposed behind the (first) working electrode for measuring carbon monoxide.
If this commercially available compound gas sensor is to be operated without microprocessor controlled compensation electronics which are additionally offered by the manufacturer, it must be calibrated with carbon monoxide or hydrogen calibration gas, if necessary at different temperatures. This, of course, is troublesome and time consuming.